Installing an RO water filter under sink takes an afternoon and a small set of tools, and the work divides into four connections: cold water in, drain out, tank, and faucet. Do them in that order, test for leaks at each stage rather than all at the end, and give the system a proper startup flush before anybody drinks from it.
Before you cut anything, plan the cabinet
Empty the cabinet and lay the system out dry. The manifold or filter bank needs to hang where you can reach the cartridges without removing the tank, the tank needs to stand upright on a flat base, and the tubing runs need gentle bends. Under sink cabinets also contain a garbage disposal, a trap and sometimes a dishwasher line, so check what space is genuinely free with the doors closed. Fit shutoff valves you can actually reach, because you will use them at every filter change.
The cold water connection
Turn off the cold angle stop under the sink and open the faucet to relieve pressure. The feed comes from the cold supply only, taken with a proper tee adapter that fits between the angle stop and the faucet supply line. Avoid the self piercing saddle valves sold in cheap kits; they clog, they weep, and many codes do not accept them. Once the tee is on, thread on the feed valve, connect the tubing, and open the water slowly with a towel under the joint so a drip is obvious before it becomes a puddle.
The drain saddle and where it must go
Membrane reject water goes to the sink drain through a saddle clamp on the tailpiece. Position it above the trap and, where the pipe is horizontal, on the top or side rather than the bottom so debris does not pack into the port. It must be upstream of the trap so sewer gas cannot enter the tubing, and it should be well clear of a garbage disposal outlet, because food debris blocking the drain line is a common cause of a system that will not fill. Drill through only one wall of the pipe and deburr the hole.
Tank, faucet and first startup
Connect the tank with its own valve so you can isolate it, and stand it where you can reach the air valve later. Fit the faucet, run the product line, and then follow the maker's startup sequence: open the feed, let the system fill the tank, then drain the tank completely, usually two or three times, before drinking any of it. That flush carries away carbon fines and any manufacturing residue from the membrane preservative. Skipping it produces water that tastes wrong and unnecessary worry about a system that is actually fine.
Leak checking and living with it
Push fit connections seat fully or they leak, so push each tube in until it stops, then pull back gently to confirm the collet has gripped, and use the locking clips supplied. Check every joint after fifteen minutes, again after a day, and put a leak sensor in the cabinet. Log the install date on the housings with a marker so future filter changes are not guesswork. Our under sink filter installation guide covers the same sequence for simpler carbon systems, which share most of these steps minus the drain.
Questions people ask about ro water filter under sink
Can I install an under sink RO system myself?
Most homeowners can, since the connections are push fit and no soldering is involved, though drilling a faucet hole in stone is worth handing to a fabricator.
Why should I avoid a saddle valve on the feed line?
Self piercing saddle valves clog with the small opening they cut, weep at the gasket over time, and are not accepted under many local plumbing codes.
Where does the drain line connect?
To a saddle clamp on the sink tailpiece above the trap, positioned away from a garbage disposal outlet so food debris cannot block the small port.
Do I need to flush a new RO system before drinking?
Yes. Fill and drain the tank several times to clear carbon fines and membrane preservative, or the first water will taste wrong.